Generally, within an automatic transmission, a control system may control a manual valve and a fluid-pressure control valve, as disclosed in a patent document 1 (i.e., Japanese patent No. 5151543). The manual valve switches a gearbox hydraulic circuit to control an operation fluid which acts on friction engagement elements such as a clutch or a brake. The fluid-pressure control valve controls a fluid pressure of the operation fluid acting on the friction engagement elements. The manual valve and the fluid-pressure control valve are controlled according to a driver operation when the driver switches the shift range.
As shown in FIG. 2, for example, when a target range is switched from the D range to the N range by an operation of a range selector, by controlling the manual valve from a D range position to an N range position for the switching of the hydraulic circuit from the D range to the N range, the fluid pressure that acts on the friction engagement element is reduced, and the friction engagement element is switched to a released-state from an engaged-state.
Then, in case that a target range is switched back to the D range from the N range, by controlling the manual valve from the N range position to the D range position, the hydraulic circuit for the N range is switched to the hydraulic circuit for the D range, together with an output of an fluid-pressure increase instruction (i.e., a specific instruction value) for the increase of the fluid pressure that acts on the friction engagement element, and the friction engagement element is switched to an engaged-state from a released-state.
In such a course of switching of the hydraulic circuits, there may be a situation when a target range is switched back to the D range immediately after switching from the D range to the N range, depending on an operation of the range selector. In such a case, as indicated by a dashed line (in the fourth row) in FIG. 2, if the manual valve is controlled to be put in a D range position and the hydraulic circuit is switched to the hydraulic circuit for the D range at a time t1 of switching of a target range from the N range to the D range, the switching of the hydraulic circuit for the N range to the hydraulic circuit for the D range occurs before the fluid pressure that acts on the friction engagement element fully decreases, which results in that the fluid pressure that acts on the friction engagement element cannot be fully lowered at a timing of such a switching of the hydraulic circuit. Thus, in such a case, since the fluid pressure control which raises the fluid pressure that acts on the friction engagement element starts in a state where the fluid pressure that acts on the friction engagement element has not fully decreased, the fluid pressure that acts on the friction engagement element may rise steeply and abruptly, which causes a shock in the automatic transmission.